Ultrasound reaction time, effect

The oxidation has also been accomplished with Claycop (montmorillonite K-10 clay supported cupric nitrate). The reaction of 96 to 102 was complete in 1.5-7 h with 81-93% yields. The time can be reduced to 5-10 minutes using ultrasound with minimal effect on yields. The major limitation of this protocol was the observation that only R = aryl gave product. Oxidation of 4-alkyl substituents was inert to these conditions with recovery of starting 96. 

The Bouveault reaction is the preparation of an aldehyde by a one pot reaction between an organic halide and lithium metal in dimethylformamide. Ultrasound has been found to markedly enhance this reaction when it is performed in tetrahydrofuran [93]. Use of an ultrasonic bath at 10-20 °C affords short reaction times of between 5 and 15 min and generates yields in the range 70-88%. Using this methodology the conversion of 1-bromobutane to pentanal (88%) can be achieved in only 5 minutes. This must be contrasted with the yield of less than 10 % which is obtained under the normal stirred conditions in the same time period. This result confirms that the effect of irradiation goes beyond mere agitation (Eq. 3.11). 

The acid catalyzed and ultrasound stimulated hydrolysis of solventless tetraethoxysi-lane-water mixtures was studied at 39°C as a function of HCl added to the mixtures (log(HCl) ranged from 0.8 to 2.0). The exothermal hydrolysis reaction causes an increasing temperature (A Tt) as a function of the reaction time, t. The isothermal hydrolysis rate constant, k, has been evaluated from the experimental (A TO versus t data, after corrections for the increasing temperature effects (Donatti and VoUet, 1996). 

The effect of ultrasound on the course of the Balz-Schiemann reaction has been studied using benzenediazonium tetrafluoroborate.263 In the presence of triethylamine trihydrofluoride in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113), fluorobenzene is formed in a 92-95% yield under 17 kHz sonication for 8 hours at 40 °C. Without ultrasound, only an 85 % yield is obtained after a reaction time of 16 hours. 

Fig. 12.8 Effect of supporting materials on sonophotocatalysis of water. Photocatalyst Ti02-B, 200 mg Light 500 W-Xe Ultrasound 200kHz, 200W Atmosphere Ar Reaction time 3 hours.

The application of ultrasound dramatically increases the rate of exfoliation of HxTi2 x/404 yH20 in the presence of aqueous tetrabutylammonium (TBA) hydroxide [130]. The effect of ultra sonication power and processing time on particle size distributions are evaluated. Applied powers of 60-300 W and reaction times of 2-30 min effectively reduce the H-Ti particle size to <100 nm. Both particle size distribution analysis and UV-Vis spectroscopy were used to study the effect of the... 

Table 3.7 Examples of the effect of ultrasound on reaction time and product yield. Source adapted from Stankiewicz [98] and Thompson and Doraiswamy [149].

Summary The efficiency of the effect of ultrasound irradiation on the reaction mixture for vinyl- and phenylchlorosilane synthesis is determined by ultrasound irradiation (USI) frequency as well as by the exposure time and the origin of starting organohalide. In the case of vinylmagnesium chloride, the formation period of the major reaction product under continuous USI exposure shortened 2.3-fold. When USI affected the synthesis during half the reaction period, the latter duration shortened by 1.4 times. In the case of phenylmagnesium chloride the process period also shortened by 2 and 1.2 times respectively for irradiation times that were 100 and 50 % of the reaction time. 

Sodium phenylselenide was prepared in 1 h by sonolysis of diphenyl-diselenide with sodium in the presence of benzopheneone, which acts as an electron transfer agent (Scheme 110) [107]. The reaction between diphenyl-diselenide and solid sodium is virtually negligible at room temperature. However, initial studies showed that the reaction could be brought to completion within four days in the presence of ultrasound. A brief investigation of the effect of solvent on the reaction was carried out in line with those described by Luche and co-workers [83]. Thus it was discovered that the reaction time could be halved by using xylene in place of THF. However, from a practical point of view, the difference in boiling points between that of xylene and THF is considerable. This would severely restrict the applicability of the method as isolation of volatile or thermally unstable selenides would be virtually impossible. 

In order to broaden the series of product available, Michael addition can be realized on acrylic esters, using imidazole (Fig. 17). Quaternization of intermediate products by nucleophilic substitution of the halogen leads to the corresponding ILs. The use of ultrasound is very effective in the 2 steps of this synthetic path since it reduces impressively reaction times. 

To find the effect of reaction temperature and ultrasoimd for the preparation of nickel powders, hydrothermal reductions were performed at 60 °C, 70 °C and 80 °C for various times by using the conventional and ultrasonic hydrothermal reduction method. Table 1 shows that the induction time, when starts turning the solution s color to black, decreases with increasing the reaction temperature in both the method. The induction time in the ultrasonic method was relatively shorter, compared to the conventional one. It assumes that hydrothermal reduction is faster in the ultrasonic method than the conventional one due to the cavitation effect of ultrasound. 

Today one of the most common chemical applications of ultrasound is the initiation of a reluctant Grignard reaction. The quantitative effects of ultrasound on the induction times for the formation of a Grignard reagent in various grades of ether is given in Tab. 3.2 [88]. 

If the yield of a silent reaction is n% after a specific period of time while the yield of the corresponding sonochemical reaction is m%, the ratio min higher than 1 is described as the effect of ultrasound. Since its beginning, ultrasound effects have been considered to originate in the general phenomenon of cavitation, which generates high temperatures, pressures, and shock waves. 

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